A spark plug is mounted to e.g. an internal combustion engine (sometimes just referred to as “engine”) and used to ignite an air-fuel mixture in a combustion chamber of the internal combustion engine. In general, the spark plug includes an insulator formed with an axial hole, a center electrode inserted in a front side of the axial hole, a metal shell arranged around an outer peripheral surface of the insulator and a ground electrode joined to a front end portion of the metal shell so as to define a discharge gap between the center electrode and the ground electrode. The metal shell has a seat portion protruding radially outwardly in a flanged shape and a male thread portion formed in front of the seat portion for mounting of the spark plug.
In some cases, a solid annular gasket is fitted around a thread neck of the metal shell between the seat portion and the male thread portion (see, for example, Japanese Laid-Open Patent Publication No. H06-283249). In a state that the spark plug is mounted to the internal combustion engine, a front end surface of the gasket located opposite to the seat portion is held in contact with the internal combustion engine (more specifically, engine head); and a rear end surface of the gasket facing the seat portion is held in contact with the seat portion. By this gasket, the air tightness of the combustion chamber can be secured.
Further, an inner peripheral portion of the gasket protrudes radially inwardly with the application of a load to an inner peripheral region of the front end surface of the gasket so that the gasket can be prevented from detachment from the metal shell. It is herein noted that a recessed portion is formed in the front end surface of the gasket with the application of such a load to the front end surface of the gasket.
In order to more assuredly prevent detachment of the solid annular gasket from the metal shell, it is desirable that the inner peripheral portion of the gasket protrudes radially inwardly by a large amount. It is thus conceivable to apply a greater load to the front end surface of the gasket for increase of the protrusion amount of the gasket. In this case, however, there is a possibility of waviness occurring on a portion of the front end surface of the gasket located adjacent to the opening of the recessed portion. The occurrence of waviness on the front end surface of the gasket results in inadequate contact between the gasket (front end surface) and the internal combustion engine in the state that the spark plug is mounted to the internal combustion engine. The air tightness of the combustion chamber may become insufficient due to inadequate contact between the gasket and the internal combustion engine. In other words, the air tightness of the combustion chamber and the detachment resistance of the gasket are in a trade-off relationship with each other. It has been difficult for the gasket to achieve both of good air tightness and high detachment resistance.
The present invention has been made in view of the above circumstances. An advantage of the present invention is a spark plug with a solid annular gasket such that, even in the case of applying a greater load for the formation of a recessed portion in the gasket, the gasket can ensure good air tightness and achieve both of air tightness and high detachment resistance.